1. Field of the Invention
The present invention relates generally to an optical transmitter used for an optical fiber communication system, and in particular, to a driver circuit suitable for an electroabsorption optical modulator or a Mach-Zehnder optical modulator and an optical transmitter employing the driver circuit.
2. Description of the Related Art
In an optical fiber communication system, due to the large capacity of transmission lines, there is an urgent demand for increasing modulation rates. In direct intensity modulation by a laser diode, a relatively large wavelength chirping limits transmission distance and modulation rate. When light signals with chirping pass through an optical fiber having chromatic dispersion (wavelength dispersion), the waveform thereof is usually distorted.
In order to prevent this problem, there is a rising expectation for the use of an external optical modulator which hardly causes chirping. As a practical external optical modulator, a Mach-Zehnder optical modulator (MZ modulator) was developed. Carrier light having a constant intensity is supplied from a light source into the MZ modulator, and intensity-modulated light signals are obtained by switching operation using light interferences. For example, there have been reported MZ modulators using LiNbO3 crystals or compound semiconductor crystals.
It has also been proposed that electroabsorption type optical modulators (EA type modulator) as external optical modulators, can be driven by lower electric power and are more suitable for size reduction than MZ modulators. The EA type modulator absorbs carrier light according to applied voltages thereby generating intensity-modulated light signals. For example, an EA type modulator using compound semiconductors has been reported.
A practical EA type modulator is provided as a semiconductor chip by semiconductor epitaxial technology. This EA type modulator can easily be integrated with a laser diode used as a carrier light source. Therefore, the reduction of coupling loss between the carrier light source and the modulator enables high-power output or miniaturization. For example, an EA-DFB laser semiconductor chip which is provided by monolithically integrating DFB-LD (distributed feedback-laser diode) with an EA type modulator, has been reported.
An optical transmitter comprising a general optical modulator and its driver circuit is described by referring to FIGS. 14 and 15. FIG. 14 is a view illustrating a circuit configuration of an optical transmitter comprising a modulator and its driver circuit, and FIG. 15 is an equivalent circuit diagram of the optical transmitter of FIG. 14.
As shown in FIG. 14, the optical transmitter X is composed of a driver circuit 100-1 which is surface-mounted in a first package, and an optical modulator (EA type modulator) 100-2 which is surface-mounted in a second package different from the first one.
The driver circuit 100-1 comprises two bipolar transistors 101-1 and 101-2 which are connected to one emitter, and respective load resistors 103-1 and 103-2, which are serially connected to the transistors. The emitter having the common connection is connected to a supply voltage VEE and current source 104-1 is provided between them. The load resistors 103-1 and 103-2 are grounded (GND). Data signals and inverted data signals are inputted into individual base terminals of two bipolar transistors 101-1 and 101-2, which act as differential amplifiers.
The optical modulator 100-2 has a p-n junction diode 102 and a resistor 110 formed by branching from the p-n junction diode 102. The differential amplifier has an output terminal 107-2 connected to an anode of the optical modulator 102, and output voltage (drive voltage) from the differential amplifier is applied to the anode of the optical modulator 102, so that input light is modulated and outputted.
In view of the foregoing, in an optical transmitter X, driver circuit 100-1 and optical modulator 100-2 are generally mounted in separate packages. There is provided a 50Ω connector 108 (Z) or a cable between the packages, and thus the driver circuit is connected to the modulator via a 50Ω line.
As described above, the optical modulator 100-2 and the driver circuit 100-1 for driving the modulator are separately packaged. As an output circuit (output stage) of the driver circuit 100-1 for the optical modulator 100-2, a collector output circuit in FIG. 14, and for example, described in JP Patent Publication (Unexamined Application) No. 11-14951 may be used. When the collector output circuit is used, the impedance matching is set at 50Ω.
As shown in FIG. 15, in the above circuit configuration, a low pass filter having a larger CR time constant is formed by a terminating resistance (R) and a capacitance (C) comprising a capacitance C12 of the EA type optical modulator 100-2 itself and other parasitic capacitances C11 such as wiring capacitance, and the CR time constant limits frequency band so as to interrupt signals of high frequencies.
In FIG. 15, for example, it is supposed that the capacitance C12 of the optical modulator is 0.5 pF and the capacitance C11 on the driver circuit side, including parasitic capacitance is 0.5 pF. Also, if the drive voltage for driving the optical modulator is 2V, and Z1 (103-2) and Z2 (110), both present in parallel, have a resistance of 50 Ω, substantial parasitic resistance is 25 Ω. Therefore, CR time constant is 25 ps. For this case, the maximum operating frequency (fm) has a limit of approximately 10 GHz. In other words, when operation is performed at a high speed (high frequency), the delay time required for signal switching between “0” and “1,” that is rising edge/falling edge, becomes large, and thus it becomes unlikely for the optical transmitter to operate normally.
In the above circuit, the R value is fixed because 50 Ω matching is necessary. Therefore, C is required to be smaller to make CR time constant smaller. Although a smaller capacitance C12 of the optical modulator is necessary to make the capacitance C smaller, the drive voltage is made to be larger than 2V when the capacitance C12 of the optical modulator is smaller. Thus, there is a trade-off between the operating frequencies and drive voltage. A larger drive voltage increases the difficulty in the high-speed operation of the driver circuit.
In addition, a smaller capacitance C12 of the optical modulator limits the power of the modulator's output light. In optical communication systems if the power of the output light is small, an optical amplifier is necessary, and thus there are drawbacks, such as that the system as a whole becomes expensive.